1. Field of the Invention
The present invention relates to reflective liquid crystal light valve projection systems and more particularly concerns improved reading light for such a system.
2. Description of Related Art
The reflective liquid crystal light valve (LCLV) is a thin film, multi-layer structure comprising a liquid crystal layer, a dielectric mirror, a light blocking layer and a photosensitive layer, all sandwiched between two transparent electrodes. In a reflective liquid crystal light valve projection system a polarized illumination (reading) beam is directed through the liquid crystal layer to the dielectric mirror which reflects it back through the liquid crystal layer. In an optically addressed system an input image of relatively low intensity writing light, such as that generated by a cathode ray tube, is applied through one of the electrodes to the photosensitive layer. Impedance of the photosensitive layer is lowered in proportion to intensity of incident writing light, resulting in a spatially varying impedance pattern. This causes a corresponding increase in voltage dropped across the liquid crystal layer in a spatially varying pattern matching the incident writing image. Tilt of the liquid crystal molecules in a particular region, and therefore the birefringence seen by the high intensity reading light passing through the region, is directly dependent on voltage dropped across the liquid crystal layer. To read the birefringence pattern, a fixed beam of linearly polarized illumination (reading) light from a high power light source floods the output face of the liquid crystal layer, passes through the liquid crystal layer and is reflected from the dielectric mirror to be polarization modulated in accordance with the input (writing) light information incident on the photosensitive layer. Therefore, if a complex distribution of light, for example a high resolution input image from the cathode ray tube, is focused on the photosensitive surface, the device converts the relatively low intensity input image into a high intensity replica image which can be reflected for projection with magnification to produce a high brightness image on a large viewing screen.
Projection systems of this type are described in several U.S. Patents, including U.S. Pat. Nos. 4,650,296 to Koda et al for Liquid Crystal Light Valve Color Projector, 4,343,535 to Bleha, Jr. for Liquid Crystal Light Valve, 4,127,322 to Jacobsen, et al for High Brightness Full Color Image Light Valve Projection System, and 4,191,456 to Hong, et al for Optical Block for High Brightness Full Color Video Projection System.
In the liquid crystal light valve projection system a significant amount of power is used by the high intensity reading light source. In the prior art, the light source provides a fixed area reading illumination that covers the entire area of liquid crystal. This high intensity reading light is not employed with optimum efficiency nor optimum contrast. In prior systems the incoming reading light beam frequently has a circular area, whereas the active area of the liquid crystal light valve usually has a rectangular configuration with an aspect ratio, for example, in the order of 16:9 in some systems. Therefore significant parts of the reading light are wasted because they fall on inactive areas. Further, in an optically addressed liquid crystal light valve the writing light input is provided, line by line, in a rectangular raster scan (by a standard CRT scan). Accordingly, a major amount of reading illumination continues to impinge upon various areas of the liquid crystal after a line of information of the raster scan has been written. The effect of the optically written input information, which is written line by line in the conventional raster scan, decreases with time after the individual line is energized. Consequently, continued application of high power, high intensity reading light decreases in efficiency with time following the writing of the input information. In many projectors, maximum allowable light input intensity is limited by allowable light valve temperatures so that the overall output intensity of the projected image may be limited unnecessarily by inefficient use of the high intensity reading light.
The above-identified related application of Alan R. Henderson and Richard M. Filia solves a number of these problems by providing a high intensity reading light that illuminates only part of the active area of the liquid crystal. In effect, a narrow beam of high intensity reading light is caused to scan the active face of the liquid crystal in synchronism with the input writing scan. The scanning of the reading light is provided in the application of Henderson and Filia by a train of quasi-cylindrical light bending or light reflecting elements that are sequentially interposed between the high intensity reading light source and the liquid crystal. The quasi-cylindrical light bending elements are mounted on a circular wheel and themselves have a circular shape. The wheel is rotated to sequentially interpose the successive bending elements between the light source and the liquid crystal to cause a narrow elongated band of light to scan in synchronism with the input scan.
Because of the curvature of the quasi-cylindrical elements of the related application, some degree of undesirable lateral scan, orthogonal to the direction of the intended scan, is provided. Because the apparatus of the related application scans by employing an angularly deflected beam, telecentric behavior of the beam is lost. Telecentric behavior is desirable wherever it is important for the beam as a whole to be perpendicular to an object or image plane, such as in a liquid crystal projection system. Further, it is desirable to enable the apparatus to be packaged in a smaller space, to further reduce any chromatic aberration, and to eliminate vertical re-trace time as the scanning shifts from one element to another in the train of quasi-cylindrical elements of the Henderson and Filia apparatus.
Accordingly, it is an object of the present invention to provide scanning illumination for an optically addressed liquid crystal light valve video projector that improves upon the arrangement described in the Henderson and Filia application.